What symptom at the onset indicates a possible subarachnoid hemorrhage?

The indication of a possible subarachnoid hemorrhage is characterized by a sudden and severe headache, often described as a "thunderclap headache" or "worst headache ever." This intense headache typically occurs suddenly and is usually accompanied by other symptoms, such as loss of consciousness, nausea, or neck stiffness. The abrupt onset and intensity of the headache are crucial indicators that differentiate it from other types of headaches, which may be gradual or less severe.

In contrast, mild headache, slight dizziness, and moderate neck pain may suggest other medical issues but are not definitive signs of a subarachnoid hemorrhage. These symptoms can manifest in many common conditions and do not have the specific urgency or characteristics displayed in the severe headache linked to hemorrhagic events. Thus, the combination of a sudden severe headache and loss of consciousness is a classic presentation that necessitates immediate medical evaluation.